This invention relates to improved methods and apparatus for safe-arm devices, and more particularly to methods and apparatus for disarming and arming oil well perforating instruments.
It has become common practice in the completion of oil and gas wells to perforate the well casing to bring the well into production by the utilization of detonating explosives of high velocity and of the general character and form known as "shaped charges", as for example, the type described in U.S. Pat. No. 2,399,211, issued to C. O. Davis et al.
In the operation of a shaped charge perforator gun, a tubular gun body containing a plurality of charges is lowered into the wellbore. The gun is positioned opposite the subsurface formation to be perforated. A current is passed from the surface through the blasting cap thereby causing explosion of the cap. The exploding blasting cap ignites a primacord which in turn causes the shaped charge to detonate. The detonated shaped charges forms a hot stream of high pressure gasses and high velocity particles, or a so called "jet", which perforates the well casing and subsurface formation.
Due to the explosive and dangerous nature of shaped charges perforating guns, when in storage or shipment the blasting cap must be segregated from the shaped charge perforating gun. In the past only by such segregation could one be assured that the perforating gun would not be accidentally fired by spurious ignition of the blasting cap due to physical shock or static electricity. Abiding by the segregation of shaped charge from blasting cap results in the necessity of assembling the perforating gun once it arrives at the well site. Due to the adverse conditions of the oil field, the technical complexities involved and the time limitations of such an operation, well site assemble has proven impractical.
These and other disadvantages are overcome with the present invention by providing method and apparatus for reliably disarming shaped charged perforating guns while in storage and shipment yet providing a simple and easy means for arming the device at the well site.